- Place back your backup (to remove the vehicles you've installed)
- Open up SparkIV (download it if you haven't got it)
- Hit browse
- Navigate to pc/models/cdimages and open up vehicles.img
- Hit the big import button
- Select the files that came with the car (probably a .wft and a .wdd or something like that)
- Hit 'Save'
- Exit SparkIV

Now try the vehicle ingame to see if it works, if it does keep installing vehicles this way. If not, then make sure you don't have any other mods that are causing this error.